Enhanced Desorption and Absorption Properties of ... - ACS Publications

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Enhanced Desorption and Absorption Properties of Eutectic LiBH4 Ca(BH4)2 Infiltrated into Mesoporous Carbon Hyun-Sook Lee,† Young-Su Lee,*,† Jin-Yoo Suh,† Minwoo Kim,§ Jong-Sung Yu,§ and Young Whan Cho† †

High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Department of Advanced Materials Chemistry, WCU Research Center, Korea University, 208 Seochang, Jochiwon, Chungnam 339-700, Republic of Korea

§

ABSTRACT: We present enhanced hydrogen storage properties of the eutectic LiBH4 Ca(BH4)2 composite confined inside a mesoporous carbon scaffold via melt infiltration. The eutectic composition was examined by analyzing the enthalpy of fusion as a function of the LiBH4-to-Ca(BH4)2 ratio. Elemental analysis by energy dispersive X-ray spectroscopy and phase analysis by X-ray diffraction patterns provided both direct and indirect evidence for the infiltration of the LiBH4 Ca(BH4)2 composite into the mesoporous channels of the carbon. The major dehydrogenation event occurred at ∼300 °C, which is lower compared with the same composite without carbon or previously reported pure LiBH4 confined in mesoporous carbons. Without additional catalytic additives, almost half of the initial hydrogen capacity was recovered after one cycling. This improved performance is a synergistic effect of the nanoconfinement and the mutual destabilization between LiBH4 and Ca(BH4)2, and it also indicates that the eutectic melting phenomenon can be exploited as an effective way of coinfiltration of reactive hydride composites.

1. INTRODUCTION Lithium borohydride, LiBH4, has attracted great attention as one of promising hydrogen storage materials due to its high theoretical hydrogen content of 18.5 wt %. The reversible hydrogen storage is governed by the equilibrium, LiBH4 T LiH + B + 1.5H2, which accounts for the actual capacity of 13.9 wt %. The hydrogen partial pressure reaches 1 atm at 370 °C.1,2 This rather high thermal stability and even more demanding rehydrogenation conditions due to slow kinetics3 have restricted its immediate application. Thus, much effort has focused on lowering the dehydrogenation temperature and improving the reversibility: addition of catalysts,4,5 partial substitution of the cation,6 destabilization with metals or hydrides,7 11 combination with less stable borohydrides M(BH4)n (M = Zn,12 Al, Zr,13 K,14 Sc,15 Ca,16 and Mg17), and confinement in nanoporous scaffolds18 29 have been tried so far. Especially, the nanoscale effect on kinetics and thermodynamics of pure LiBH4 has been extensively studied recently18 29 under the hope for faster H-exchange kinetics and thermodynamic destabilization via size reduction.30 For fabrication of nanocrystalline hydrides, a conventional ball-milling technique can be used, but repeated dehydrogenation and rehydrogenation cycles at elevated temperatures naturally cause grain growth and particle agglomeration.31 To physically suppress the size growth, mesoporous or nanoporous carbon scaffolds have been preferably adopted for size confinement due to their chemical inertness and light weight. The size confinement indeed brings about certain improvement: the dehydrogenation temperature of r 2011 American Chemical Society

LiBH4 incorporated into various carbon scaffolds decreased to 300 380 °C from 450 500 °C for pure LiBH4.22 28 Although there are a number of hydrogen storage materials that outperform pure LiBH4 in terms of dehydrogenation temperature and reversibility,7,10,32 34 the reason why so much attention has been given especially to the nanoconfined LiBH4, in our opinion, lies partly in the simplicity of the melt infiltration of LiBH4 into carbon scaffolds at a relatively low temperature: LiBH4 melts before the temperature reaches the dehydrogenation point. For instance, another prototypical hydrogen storage material, MgH2, requires a more complicated chemical process or rather high temperature above the melting point of magnesium, since it remains as a solid until it releases hydrogen.35,36 In this respect, the recently investigated mixed x)Ca(BH4)2, which borohydride composite, xLiBH4 + (1 exhibits eutectic melting, offers a unique opportunity for nanoconfinement. The two constituents react with each other, resulting in lower dehydrogenation temperatures than both pure LiBH4 and Ca(BH4)2.16 On top of that, the composite can be fully melt-infiltrated at ∼200 °C, which is even lower than the melting point of pure LiBH4, if x corresponds to the eutectic composition. In this regard, we investigated the synergistic effect on the hydrogen storage properties of the eutectic xLiBH4 + (1 x)Ca(BH4)2 composite nanoconfined in mesoporous carbon Received: June 26, 2011 Revised: August 26, 2011 Published: August 26, 2011 20027

dx.doi.org/10.1021/jp206000h | J. Phys. Chem. C 2011, 115, 20027–20035

The Journal of Physical Chemistry C scaffolds (henceforth labeled as LCC) in comparison with pure LiBH4. This is, to the best of our knowledge, the first report on the nanoconfinement effect of a binary borohydride mixture coinfiltrated into mesoporous scaffolds. We started by locating the eutectic composition of xLiBH4 + (1 x)Ca(BH4)2 from the enthalpy of fusion and X-ray diffraction (XRD) patterns. The composites were infiltrated into the nanosized pores by melting, which is an easy and simple method compared with a chemical impregnation.25 The infiltrated composites inside the mesopores were confirmed by highangle annular dark field image produced in a scanning transmission electron microscope (STEM-HAADF) and energy-dispersive X-ray spectroscopy (EDX) line profile, which serves as direct and indisputable evidence for the infiltration. The hydrogen desorption process of the eutectic composite physically mixed with carbon was analyzed with differential scanning calorimetry (DSC), thermogravimetry (TG), and XRD. For the same composite incorporated into the mesoporous carbon, the temperature of major dehydrogenation reaction was prominently lowered to ∼300 °C. About 50% of initial hydrogen was recharged reversibly without any additional catalytic additives. It was clearly demonstrated that desorption and absorption properties of the eutectic LiBH4 Ca(BH4)2 composite infiltrated into the carbon were enhanced compared with those of pure LiBH4.

2. EXPERIMENTAL SECTION 2.1. Mesoporous Carbon. 2.1.1. Synthesis of SBA-15 and CMK-3. The mesoporous rod-type SBA-15 silica particles were

synthesized on the basis of a previously reported procedure.37 CMK-3 was fabricated by replication through nanocasting of SBA-15 silica and using phenol as the carbon source. Aluminum was incorporated into the silicate framework through an impregnation method to catalyze polymerization of phenol and paraformaldehyde. The detailed procedure for the aluminum impregnation and the nanocasting is described elsewhere.37 2.1.2. Characterization. Powder XRD patterns at small angles were obtained with a Rigaku 1200 using Cu Kα radiation source. After the CMK-3 was degassed at 150 °C to 20 μTorr for 4 h, nitrogen adsorption and desorption isotherms were measured at 196 °C on a KICT SPA-3000 gas adsorption analyzer. The Brunauer Emmett Teller (BET) method was used to estimate the specific surface areas from N2 adsorption data. The total pore volume was evaluated from the adsorbed amount of gas at the relative pressure of 0.99. Pore size distribution was derived from the analysis of the adsorption branch based on the Barrett Joyner Halenda (BJH) calculation method. Scanning electron microscopy (SEM) images and high-resolution SEM images were obtained using Hitachi S-4700 and Hitachi S-5500 microscopes with an acceleration voltage of 10 kV and 30 kV, respectively. 2.2. LiBH4 Ca(BH4)2 Mesoporous Carbon. 2.2.1. Sample Preparation. Commercial LiBH4 (assay 95%, Acros) and Ca(BH4)2 (assay 98%, Sigma-Aldrich) were used without further purification. The mesoporous carbon was dried at 420 °C under vacuum for 6 h. Three different samples were prepared: (1) ballmilled LiBH4 Ca(BH4)2 composites, (2) sample 1 physically mixed with the porous carbon using the ball-milling process (asmilled LiBH4 Ca(BH4)2 carbon (LCC)), and (3) sample 2 infiltrated into the carbon pores by melting (infiltrated LCC). According to the previous in situ XRD investigation of the

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composites xLiBH4 + (1 x)Ca(BH4)2, the eutectic composition was narrowed down to the range of x = 0.6 0.8.16 To further x)Ca(BH4)2 refine the eutectic composition, xLiBH4 + (1 mixtures were prepared by ball-milling in different molar ratios of x = 0.50, 0.60, 0.65, 0.68, 0.70, 0.75, and 0.80. The eutectic composition of the binary mixture was determined from calorimetric analysis and then confirmed by XRD. For the as-milled LCC sample, the amount of carbon was chosen such that its mesopore volume is equal to the volume of the LiBH4 Ca(BH4)2 composite. The ball-milling was performed for ∼1 g of the mixture with three 12.7 mm and seven 7.9 mm diameter Crsteel balls using a planetary mill (Fritsch P7) operated at 600 rpm for 1 h. For the preparation of the infiltrated LCC, the as-milled LCC powder was heated at 230 °C for 30 min under 3 bar of hydrogen pressure. The temperature was chosen to be over the eutectic melting point of LiBH4 Ca(BH4)2 (∼200 °C) to ensure full melt-infiltration. All sample preparations were performed in an argon-filled glovebox (LABstar, MBraun, p(O2, H2O)< 1 ppm). 2.2.2. X-ray Diffraction. In situ synchrotron XRD measurements were performed at the 10B 3 XRS KIST-PAL beamline in Pohang Accelerator Laboratory. The selected X-ray wavelength was 1.00164 Å. Each sample was loaded into a sapphire tube (o.d. = 1.52 mm, i.d. = 1.07 mm) in the argon-filled glovebox. To investigate the dehydrogenation procedure, the temperature was raised at a rate of 2 °C/min from room temperature to 560 °C under a constant pressure of p(H2) = 3 bar. During the in situ experiment, XRD patterns were collected every 228 s with a MAR345 image plate detector. The 2D images were converted into 1D diffraction data by the FIT2D program.38 Ex-situ XRD measurements were also conducted on the prepared samples sealed in glass capillary tubes using a Bruker D8 advance diffractometer with Cu Kα radiation. 2.2.3. Electron Microscopic Analysis. Infiltration of the borohydride composites into the mesopores was confirmed by STEM-HAADF image and EDX line profile. The sample for TEM observation was taken from the borohydride infiltrated carbon and was prepared by FEI Qunata3D focused ion beam (FIB). To avoid air contact during the sample preparation, an airlock transfer device was used to transfer the sample from the glovebox to the FIB equipment and vice versa. The prepared TEM sample grid was installed on a Gatan double tilt vacuum transfer holder, model 648, again to avoid air contact during transfer from glovebox to TEM. FEI Titan 80-300 was used with 300 kV extraction voltage for the TEM observation. 2.2.4. Thermal Analysis. To find the eutectic composition of x)Ca(BH4)2, the enthalpy of fusion was xLiBH4 + (1 investigated for various ratios of x utilizing a differential scanning calorimeter (DSC 204 F1, Netzsch) under flowing argon (99.9999% purity, 40 mL/min). The hydrogen desorption and absorption behavior was analyzed by a high-pressure differential scanning calorimeter (HP-DSC, DSC 204 HP, Netzsch) and a thermogravimeter (TG 209 F1, Netzsch) under flowing hydrogen (99.9999% purity, 50 mL/min) and argon (99.9999% purity, 40 mL/min), respectively. For the DSC, HP-DSC, and TG measurements, the samples sealed into an aluminum pan with a lid were heated up to 500 °C at a ramping rate of 5 °C/min.

3. RESULTS AND DISCUSSION 3.1. Characterization of Mesoporous Carbon. Figure 1a and b shows representative SEM images of CMK-3 carbon at 20028

dx.doi.org/10.1021/jp206000h |J. Phys. Chem. C 2011, 115, 20027–20035

The Journal of Physical Chemistry C

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Figure 1. (a) SEM image, (b) high-resolution SEM image, (c) XRD pattern, and (d) N2 isotherms and corresponding pore size distribution (inset) of CMK-3.

Table 1. Surface Structure Parameter for SBA-15 and Corresponding CMK-3 d spacing (nm)

a d

unit cell parameter a0 (nm)a

SBET (m2g 1)b

Vtotal (cm3g 1)c

Vmicro (cm3g 1)c

Vmeso (cm3g 1)c

pore size (nm)d

SBA-15

9.69

11.19

718

1.42

0.32

1.10

7.6

CMK-3

9.13

10.54

1229

1.63

0.59

1.04

3.5

XRD unit cell parameter equal to a0 = 2 d(100)/31/2. b The uncertainty of SBET is (20 m2 g 1. c The uncertainty of pore volume is (0.05 cm3 g 1. Maximum value of the BJH pore size distribution peak was deduced from the adsorption branch of the N2 isotherm.

different magnifications. The SEM image in Figure 1a shows a monodisperse distribution for rodlike CMK-3 carbon. The morphology and mesopore structures can be also evidenced by the HR-SEM image shown in Figure 1 b. The CMK-3 particle possesses uniform rodlike morphology, around 1.5 μm in length and 0.59 μm in diameter. Figure 1c illustrates the powder XRD pattern of the rodlike mesoporous CMK-3 sample. The powder XRD patterns exhibited primarily a very intense (100) diffraction peak centered at 2θ = 1° along with weaker (110) and (200) reflections characteristic of a 2D hexagonal structure. The mesostructural character of the CMK-3 was investigated by nitrogen adsorption desorption isotherms. The isotherms and the pore size distribution curve of the CMK-3 sample are shown in Figure 1d. A type IV isotherm with an H2-type hysteresis loop characteristic of mesoporous materials with channel-type pores was observed for the sample. From the pore size distribution curve (inset) derived from the adsorption branch, the pore size of CMK-3 is estimated to be ∼3.5 nm. The BET surface area and mesopore volume of CMK-3 are 1229 ( 20 m2/g and

1.04 ( 0.05 cm3/g, respectively. Surface structural parameters for the CMK-3 along with the parent SBA-15 silica are summarized in Table 1. 3.2. Eutectic Composition of xLiBH4 + (1 x)Ca(BH4)2. In a binary mixture, the molar enthalpy change for the eutectic melting varies linearly with the composition and becomes maximum at the eutectic composition.39 The enthalpy change is proportional to the area of the endothermic or exothermic peak obtained by DSC. Figure 2a presents the DSC curves for various mixtures of xLiBH4 + (1 x)Ca(BH4)2 (henceforth noted as xL(1 x)C) with different x’s. As Figure 2a shows, the eutectic melting occurs around 200 °C for all mixtures, except for pure LiBH4 (x = 1) and Ca(BH4)2 (x = 0), as previously reported.16 The enthalpy for the eutectic melting was obtained from the peak area estimated by means of numerical integration using a linear baseline. This was done for several measurements at the same composition, as indicated with different symbols in Figure 2b. The maximum enthalpy change is found in the range of 0.65 < x < 0.70. To further refine the eutectic composition, we checked the x)Ca(BH4)2 composites XRD patterns of xLiBH4 + (1 20029



Figure 2. (a) DSC curves for binary xLiBH4 + (1 x)Ca(BH4)2 composites with different molar ratios of x/(1 x), which were measured at a heating rate of 5 °C/min under flowing argon. The eutectic melting peak is commonly observed at around 200 °C. (b) Determination of the eutectic composition of xLiBH4 + (1 x)Ca(BH4)2 by using a simple relation between the enthalpy of the eutectic peak and the mole fraction of x. Eutectic composition is in the range of x = 0.65 0.7, at which the enthalpy becomes maximum. The different symbols in part b indicate several measurements at the same x. The error bars show uncertainty in the peak area at each measurement.

infiltrated into the carbon pores near the composition of x = 0.65 0.70. Figure 3a presents the diffraction pattern of the porous carbon. The very broad peaks at around 2θ = 22, 45, and 79°, which are related to the (002), (101), and (110) of a graphitic structure, show the material is rather close to being amorphous. The infiltrated 0.68L0.32CC in part d, where x)Ca(BH4)2 + C xL(1 x)CC denotes xLiBH4 + (1 composite, does not show any Bragg peaks from either LiBH4 or Ca(BH4)2, whereas the as-milled 0.68L0.32CC in part c shows those peaks as in part b. This indicates that both LiBH4 and Ca(BH4)2 in 0.68L0.32CC fully melted during heating up to ∼230 °C and were infiltrated into the carbon mesopores. No diffraction peak in the infiltrated 0.68L0.32CC sample indicates the confined composite within the pores becomes nanocrystalline or amorphous. On the other hand, the XRD pattern of the infiltrated 0.7L0.3CC in part e exhibits the peaks of o-LiBH4, which indicates remaining unmelted LiBH4, even above the eutectic melting temperature. Therefore, we conclude that the eutectic composition of xLiBH4 + (1 x)Ca(BH4)2 lies very close to x = 0.68. 3.3. Electron Microscopic Confirmation of Infiltration. The XRD patterns in Figure 3 provide only indirect evidence that the

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Figure 3. XRD patterns of (a) mesoporous carbon, (b) 0.68LiBH4 + 0.32Ca(BH4)2 composite (0.68L0.32C), (c) as-milled 0.68LiBH4 + 0.32Ca(BH4)2 composite with the porous carbon (as-milled 0.68L0.32CC), (d) infiltrated 0.68LiBH4 + 0.32Ca(BH4)2 composite into the porous carbon (infiltrated 0.68L0.32CC), and (e) infiltrated 0.7LiBH4 + 0.3Ca(BH4)2 composite into the porous carbon (infiltrated 0.7L0.3CC). No Bragg peaks in part d indicates that the composite infiltrated into the porous carbon pores is amorphous. The eutectic composition is determined to be closer to x = 0.68 than x = 0.7.

borohydride composite would exist in mesoporous channels after infiltration. We proceeded one step further to find direct evidence of infiltration with electron microscopy. Figure 4a shows STEM-HAADF image of the infiltrated sample, which exhibits a well-defined structure of the porous channels in the carbon. The EDX scans along the vertical line shown in part a are displayed in part c for Ca and in part d for C. The intensity was calculated by integrating the shaded region of the drift corrected EDX spectrum profile shown in part b. The intensity line profiles show maximum counts of Ca and minimum counts of C at the same position marked with a dotted line, which coincides with the bright line in the image of part a. Therefore, the bright lines originate from larger amount of the heaviest element, Ca, and correspond to the porous channels containing the borohydride composite. The periodic patterns in the image and in the line scans are in accord with each other and indicate highly ordered arrays of channels in the carbon. 3.4. Desorption Properties. 3.4.1. Eutectic 0.68LiBH4 + 0.32Ca(BH4)2 Composite. Dehydrogenation performance of the samples was examined with HP-DSC and TG. Temperatures at which a noticeable weight loss occurs were determined by the first derivative of the weight vs temperature curves. The results of the eutectic 0.68L0.32C composite are presented in Figure 5. As can be seen in the HP-DSC plot in part a, eutectic melting appears at around 200 °C. Below 200 °C, the well-known polymorphic phase transitions are observed at around 117 °C 20030



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Figure 4. (a) STEM-HAADF image of the infiltrated 0.68LiBH4 + 0.32Ca(BH4)2 composite into the porous carbon. (b) EDX spectrum profile along the vertical line marked in part a. (c, d) Intensity as a function of position obtained from the EDX line scans for Ca and for C. The results indicate that the composite is infiltrated inside the highly ordered array of porous channels, which appear as the bright lines in part a.

for orthorhombic to hexagonal LiBH4 transformation and at around 143 °C for α- and γ-Ca(BH4)2 to β-Ca(BH4)2 phase transition.16 Between eutectic melting (∼200 °C) and the main desorption (∼400 °C), endothermic and exothermic peaks are observed near 260 and 330 °C, respectively. The origin of those peaks was identified by analyzing the reaction path of the preheated compounds above the eutectic melting temperature with ex-situ XRD. For this, each sample was prepared by holding at the desired temperature for 30 min under p(H2) = 3 bar and cooling subsequently to room temperature. The XRD results of the preheated sample are displayed in Figure 6. At 230 °C (part a), some amount of Ca3(BH4)3(BO3)40 and a lesser amount of LiCa3(BH4)(BO3)241 come into view because it is difficult to avoid impurities. Upon reaching 260 °C (part b), the Ca3(BH4)3(BO3) phase disappears and the LiCa3(BH4)(BO3)2 phase increases. The formation of these two borohydride borate phases accompanies a weight loss near 250 °C, that is, hydrogen release, as shown in the TG curve in Figure 5b. Combining the above results, we can infer that the borate groups, [BO3]3 , are produced by oxidation of [BH4] in LiBH4 Ca(BH4)2, causing hydrogen release. Therefore, the minor endothermic DSC peak around 260 °C might originate from the oxidation of [BH4] in the molten LiBH4 and Ca(BH4)2, not from a polymorphic phase transition. The amount of LiCa3(BH4)(BO3)2 increases from part c to part d, which might be responsible for the broad exothermic behavior in DSC at around 330 °C, as shown in Figure 5a. The main dehydrogenation of the eutectic LiBH4 Ca(BH4)2 composite contributes to the maximum endothermic peak at ∼400 °C in DSC (Figure 5a). This can be supported by the major weight loss near ∼400 °C in TG (Figure 5b) and the appearance of CaH2 in the XRD pattern in Figure 6e. The disappearance of LiCa3(BH4)(BO3)2 in XRD at ∼465 °C (Figure 6f) implies that there is no majority phase containing [BH4] above this temperature, and thus, no abrupt change in DSC and TG plots can be seen in Figure 5.

3.4.2. 0.68LiBH4 + 0.32Ca(BH4)2 with Porous Carbon. The HP-DSC and TG results of the as-milled and the infiltrated 0.68L0.32CC samples are displayed in Figure 7a and b, respectively. In the DSC curve of the as-milled 0.68L0.32CC, only the structure change of LiBH4 is observed, and the signals for α- to βCa(BH4)2 phase transition and for the eutectic melting are not seen in contrast with LiBH4 Ca(BH4)2 (Figure 5a). Instead of those, a broad exothermic peak suggesting the wetting of molten LiBH4 Ca(BH4)2 composite into the pores shows up. The exothermic signature for wetting was previously reported for the physical mixtures of LiBH422 or NaAlH442 with nanoporous carbon. However, in those mixtures, the endothermic peak corresponding to the melting of the compound always preceded the exothermic peak due to wetting.22,42 According to our DSC results, we can tell that the endothermic enthalpy change due to the eutectic melting is smaller compared with the exothermic enthalpy change due to the wetting of the carbon surface in the case of 0.68L0.32CC. This can be compared with the DSC curves of 0.75L0.25C and as-milled 0.75L0.25CC shown in the inset of Figure 7a. Here we can still find the endothermic peak at around 200 °C, even when the porous carbon is added, but its size becomes significantly reduced. It seems that which one brings larger enthalpy change between melting and infiltration depends on the composition. In addition, melting and infiltrating or wetting of molten LiBH4 Ca(BH4)2 composite proceed almost at the same time such that each process could not be distinguished within our measurement condition. In the case of the as-milled sample, weight loss starts to be seen right after melt infiltration. We think that it is mainly due to the formation of borate anions, as discussed before. This reasoning is supported by the fact that the infiltrated sample that is pretreated at 230 °C releases a lesser amount of hydrogen at this temperature range, since part of the borate anions were already formed during pretreatment. Both the as-milled and the infiltrated 20031



Figure 5. (a) HP-DSC and (b) TG traces of the eutectic 0.68LiBH4 + 0.32Ca(BH4)2 composite. The temperature at which a great weight loss occurs due to hydrogen release is determined by a peak position at the first derivative of the TG curve.

0.68L0.32CC samples undergo a major dehydrogenation reaction at ∼308 °C, which is quite lower than the ∼405 °C for 0.68L0.32C (Figure 5b) as well as the ∼450 500 °C1,2 for pure LiBH4. Thus, the nanoconfinement indeed brings a positive effect. Whether it is due to a change in thermodynamics or kinetics is not clear at this point and is the subject of future study. We would like to mention that the infiltrated sample of 0.68L0.32CC does not show any of structural transition or melting in the DSC curve in Figure 7a. This is in agreement with the result of premelted LiBH4 nanoporous carbon22 and means that the nanoconfined LiBH4 Ca(BH4)2 composite would be in the amorphous state. We measured FT-IR spectra (not shown here) of the composite to probe [BH4] anions inside the carbon, but the signal was screened by carbon and did not reveal any useful information. Other analytical tools such as NMR spectroscopy21,23 seem to be more capable of elucidating the status of the nanoconfined [BH4] or cations, and experiments are underway in this direction. The dehydrogenation path of the as-milled and the infiltrated 0.68L0.32CC samples was investigated by ex-situ XRD. As can be seen in Figure 8a, the as-milled sample preheated at 230 °C and resolidified at room temperature shows only broad peaks from the porous carbon. We can deduce that most of the composite was infiltrated into the pores. As the temperature is increased above 250 °C, the XRD peaks for LiCa3(BH4)(BO3)2 phase come out, and the peak intensity increases with temperature. The overall sequence is similar to that of 0.68L0.32C in Figure6, except for the fact that we cannot observe LiBH4 and Ca(BH4)2 in Figure 8a, since they are in the amorphous state.

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Figure 6. Ex-situ XRD patterns of the eutectic 0.68LiBH4+0.32Ca(BH4)2 composite at each step of dehydrogenation: (a) 230, (b) 260, (c) 300, (d) 350, (e) 400, and (f) 465 °C.

The rather sharp XRD peaks of LiCa3(BH4)(BO3)2 tell us that this phase must exist outside the mesoporous channels, since the average channel diameter is only two times larger than the unit cell of LiCa3(BH4)(BO3)2.41 Therefore, we could not exclude the possibility that some portion of the borohydride composite is covering the surface of the carbon particles, forming a very thin nanometer scale surface layer. Then it would not be detected during the premelting and resolidifying procedure, but it could act as the source of LiBH4 and Ca(BH4)2 for the formation of LiCa3(BH4)(BO3)2 outside the mesopores. The in situ XRD patterns of the infiltrated sample in Figure 8b also show similar results. When the temperature is increased above ∼450 °C, the LiCa3(BH4)(BO3)2 phase starts to be dissociated, probably releasing hydrogen. This can be correlated with gradual weight loss in TG curves (Figure 7b) at higher temperature in the as-milled and the infiltrated 0.68L0.32CC samples. 3.4.3. Hydrogen Storage Capacity. Total weight loss up to 500 °C is about 11 wt % for the eutectic 0.68L0.32C composite (Figure 5b), which falls within the range of the upper and lower limit16 estimated roughly. The weight loss of the as-milled 0.68L0.32CC is ∼5 wt % (Figure 7b), which is reasonable because LiBH4 Ca(BH4)2 and carbon were mixed at a weight ratio of 1:1. However, the mass loss of the infiltrated 0.68L0.32CC is lower than that of the as-milled sample. This is due to the fact that the infiltrated sample was partly dehydrogenated during preheating at 230 °C. This is supported by DSC and the TG curves in Figure 7a and b, which shows that the onset of dehydrogenation 20032


The Journal of Physical Chemistry C in the infiltrated sample shifts to higher temperature than that in the as-milled sample.

Figure 7. (a) HP-DSC and (b) TG traces of the eutectic 0.68LiBH4 + 0.32Ca(BH4)2 composites milled with and infiltrated into the porous carbon. The inset shows the HP-DSC curves of 0.75LiBH4 + 0.25Ca(BH4)2 and as-milled 0.75LiBH4 + 0.25Ca(BH4)2 + carbon.

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3.5. Absorption Property. The reversibility of the as-milled 0.68L0.32CC was tested by ex-situ XRD and TG. The sample was initially dehydrogenated by heating at 400 °C for 2 h at p(H2) = 3 bar and rehydrogenated at 400 °C for 24 h at high pressure of p(H2) = 110 bar. We would like to emphasize that rehydrogenated LiBH4 and Ca(BH4)2 in the mesoporous carbon cannot be detected by XRD. Therefore, we performed TG analysis to know the degree of reversibility. Even though the rehydrogenation was not complete, as TG data show in the inset of Figure 9, ∼50% of hydrogen contained in the as-milled sample was recovered, and the main desorption temperature remained unchanged after one cycling. Although LiBH4 and Ca(BH4)2 are not at all seen in the XRD pattern, the constant main desorption

Figure 9. Ex-situ XRD patterns of the as-milled 0.68LiBH4 + 0.32Ca(BH4)2 composite with carbon, which was initially dehydrogenated at 400 °C for 2 h under p(H2) = 3 bar and then rehydrogenated at 400 °C for 24 h under p(H2) = 110 bar. The inset shows TG data for the asmilled and the rehydrogenated samples.

Figure 8. (a) Ex-situ XRD patterns of the as-milled 0.68LiBH4 + 0.32Ca(BH4)2 with carbon. Each sample was prepared by heating at the desired temperature (230, 250, 325, or 400 °C) for 30 min under p(H2) = 3 bar and cooling down to room temperature. (b) In situ XRD patterns of the infiltrated 0.68LiBH4 + 0.32Ca(BH4)2 into carbon measured with increasing temperature at the condition of p(H2) = 3 bar. 20033


The Journal of Physical Chemistry C temperature clearly tells us that LiBH4 and Ca(BH4)2 were certainly formed inside the mesopores during rehydrogenation and were dehydrogenated according to the same reaction scheme. It is interesting to note that the weight profile of the rehydrogenated sample remains flat up to ∼250 °C, which is different from that of the as-milled sample. This again indicates that once oxygen impurities oxidize [BH4] and transform into [BO3]3 anions, they remain as they are without further consuming [BH4] . The coexistence of LiCa3(BH4)(BO3)2 and Ca3(BO3)2 phases hints that outside the mesoporous channels, rehydrogenated LiBH4 would exist in the form of LiCa3(BH4)(BO3)2 since it is more stable than isolated LiBH4 and Ca3(BO3)2.41 However, the major dehydrogenation is certainly not from LiCa3(BH4)(BO3)2, which is supposedly even more stable than LiBH4 and whose dehydrogenation proceeds between 400 and 500 °C (Figure 8b). The reversible capacity of our mixture without additional catalytic additives is very affirmative for improving hydrogen storage properties by using porous carbons.

4. CONCLUSIONS We present improved desorption/absorption properties of LiBH4 Ca(BH4)2 carbon compared with pure LiBH4, which is due to the synergistic effect of nanoconfinement inside the mesoporous carbon and the mutual destabilization of LiBH4 and Ca(BH4)2. By refining the eutectic composition as 0.68LiBH4 + 0.32Ca(BH4)2, the binary borohydride mixture could be well co-infiltrated into the pores at a temperature lower than the melting point of LiBH4. The infiltrated composite inside the mesoporous channels of the carbon was clearly detected by the electron microscopic and the XRD analysis. The major desorption temperature was significantly lowered to ∼300 °C from ∼400 °C for the bulk 0.68LiBH4 + 0.32Ca(BH4)2 composite. About 50% of the initial hydrogen capacity of the 0.68LiBH4 + 0.32Ca(BH4)2 composite mixed with carbon was recovered after one cycle without any additional catalytic additives. From the results of XRD and TG, we can conclude that the porous carbon played an important role in reducing the dimensions of the hydride particles without any chemical reaction with the hydride. It was demonstrated that nanoconfinement can surely provide enhanced hydrogen desorption/ absorption properties. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been sponsored by the Korea Research Council of Fundamental Science and Technology. The authors thank Jong-Min Kim for her technical support and expert operation of the Titan TEM, and Jae-Hong Noh and Young-Im Wang for TEM sampling using the Qunata3D FIB. The authors acknowledge the support by Dr. Gyeung-Ho Kim and Dr. Jae-Pyoung Ahn for the analytical activities using TEM and FIB and by Dr. Ji Woo Kim, Kee-Bum Kim, Dr. Ik Jae Lee, and Dr. Keun Hwa Chae for the in situ synchrotron XRD measurements. JongSung Yu is thankful for HRSEM measurements performed in KBSI.

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Enhanced Desorption and Absorption Properties of ... - ACS Publications

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